The invention pertains generally to a belt drive including two belt sheaves, a belt trained around the sheaves, and means for adjusting the tension of the belt. More particularly, this invention pertains to power shovels having a boom on which a dipper stick is mounted for pivoting about and sliding therewith, the dipper stick having a dipper at its lower end. The dipper stick is mounted for movement relative to the boom so that the dipper can be moved into and out of the bank by a crowd drive. The crowd drive works in conjunction with the hoist motion (which raises and lowers the dipper) to dig and position the resulting load of excavated material.
The crowd drive is a heavily shock loaded drive. Upon hitting a hard toe (an area not as well fragmented by a shot blast) in the bank the load on the drive system can dramatically increase. For this reason, power from a crowd drive crowd motor is typically transmitted to the gear case via two large belts consisting each of five 8V belts banded together. The purpose of the belts is to absorb the shock loads when they occur. Properly tensioned belts will slip on a sheave before they break, and limit the impact loading on the entire gear train.
The loading on this drive is nearly fully reversing, meaning that the loading is nearly the same in both crowd and retract. Because of this, the system needs to be able to handle operating loads in both directions.
There exists a need to increase the accuracy of belt tensioning and reduce the amount of time required to perform the work. Properly tensioned belts are critical to the life of the belts and the performance of the mining shovel. All past means of tensioning belts have required manual labor. Because of this, belt tensioning can mean at least one hour or more of machine downtime. This is very costly to the customer (roughly $15,000 to $20,000 in lost ore production). Most importantly, there always remained a potential for human error. The pressure gage used to set the belt tension could only be read to the nearest 50 psi. The accuracy of the settings has always been a concern.
Currently the belts, when new, are overtensioned beyond what is needed because new belts stretch very quickly. This lengthens the interval for when the first retensioning needs to occur. The current system for knowing when to retension is to listen for the belts to squeal when they slip excessively. This is a very unreliable process on current machines because, with acoustic insulation in the operator's station, he may not hear it. This requires checking by time rather than by sound. If allowed to continue to operate when slipping, belts will glaze such that normal tensioning will no longer prevent slippage. Further overtensioning of the belts can lead to shaft breakage or bearing failures.